The present invention relates to an exhaust gas recirculation system (EGR) provided for an internal combustion engine, and more particularly to an improvement in an EGR system for accomplishing the reduction in the amount of nitrogen oxides (NO.sub.x) contained in the exhaust emission from an internal combustion engine, at a low cost.
In an internal combustion engine, particularly in a car engine, exhaust gas re-circulation has been widely used as an effective method for reducing the amount of harmful NO.sub.x emitted from the car engine, since the legislative standards for limiting the amount of NO.sub.x exhausted from a car engine to a specific low level has become increasingly strict. However, those skilled in the art know that usage of said EGR method for a car engine often causes a decrease in the engine performance, since inactive exhaust gas is re-introduced into an intake system of the car engine. Therefore, if the EGR method is used for an internal combustion engine in order to ensure appropriate engine performance as well as to acquire the highest possible reduction of NO.sub.x, it is necessary to carefully regulate the amount of the re-circulating exhaust gas in response to the change in the state of operation of an internal combustion engine and/or a vehicle in which the engine is mounted. In order to meet the above requirement, an improvement of an EGR system of an internal combustion engine has already been proposed by which two separate flow-control valves are arranged so as to control the flowing amount of the exhaust gas re-introduced from an exhaust system into an intake system of an internal combustion engine in a two step-like manner, depending upon the change in the state of the engine operation. However, because of the two separate flow-control valves in the EGR system, the distribution of the EGR gas pipelines in the engine compartment and the physical structure within said engine compartment for mounting the two flow-control valves becomes very complicated. Further, the time and operation required for mounting said two flow-control valves causes a large decrease in the productivity of workers making the engine, and thus an increase in the cost for manufacturing the engines.